Microsoft has rolled out a change to all SharePoint Online tenants created on or after August 2020. As a result many third party add-ins, including the Muhimbi PDF Converter Online, show an error when using the add-in. Tenants created before this time work just fine.
Fortunately the tenant's behaviour can be changed as follows:
Make sure the latest version of the SharePoint Online Management Shell is installed. The steps below have been tested using version 16.0.20616.12000. You can verify the currently installed version using the following command:
Get-Module -Name Microsoft.Online.SharePoint.PowerShell -ListAvailable | Select Name,Version
Please note that upgrading an old Management Shell can be problematic. Please see this blog post for more details.
Launch the 'SharePoint Online Management Shell'.
Connect to your environment's tenant admin URL using the following command.
Connect-SPOService -Url https://tenant-admin.sharepoint.com
Make sure you replace the URL with your tenant name and include the '-admin' suffix. You will be asked to login, please make sure the credentials for a SharePoint admin are specified. (You may need to reach out to your SharePoint Online support team in order to make this change)
Execute the following command to make the change
Set-SPOTenant -DisableCustomAppAuthentication $false
Please note that it may take 5 minutes for this change to take effect.